1. Field of the Invention
The present invention relates generally to tubular members such as those used to form a drill stem for use in horizontal directional drilling operations and to a coupling and a coupling technique for increasing the applicable torque that such drill stem members can withstand.
2. Description of the Prior Art
In today's world, there are numerous examples of underground conduits, lines and cables which surround us and which enable many of our everyday activities. For example, there are utility lines for water, electricity, gas, telephone, cable television, digital communication and computer connections. It is oftentimes preferable to bury these lines for reasons of safety and aesthetics instead of, for example, running physical pipelines above ground or electrical lines and cables overhead. In many situations, the underground utilities can be buried in a trench, which is subsequently back-filled. Trenching is most advantageously used in areas of new construction. In areas where an existing infrastructure is already in place, however, a trench can cause serious disturbance to structures or roadways. Further, there is always the possibility that digging a trench may damage previously buried utilities, and that structures or roadways disturbed by digging the trench are rarely restored to their original condition. Even in areas of new construction, the burial of utilities in a trench has certain disadvantages. For example, the trenching operation can pose a danger of injury to workers as well as passing traffic.
For these and other reasons, alternative techniques such as horizontal directional drilling (HDD) operations, sometimes referred to as “trenchless” drilling operations, are becoming ever more popular. In the typical HDD operation, a boring machine is positioned on the ground surface. The boring machine is arranged to drill a hole into the ground at an oblique angle with respect to the ground surface. Fluid may be pumped through the drill string, over the boring tool, and back up the borehole in order to remove cuttings and dirt. After the boring tool reaches the desired depth, the tool is then directed along a substantially horizontal path to create a horizontal borehole. After the desired length of borehole has been obtained, the tool is then directed upwards to break through to the surface. A reamer may then attached to the drill string which is pulled back through the borehole, thus reaming out the borehole to a larger diameter. It is common to attach a utility line or conduit to the reaming tool so that it is dragged through the borehole along with the reamer during this step in the operation.
A typical horizontal directional drilling machine includes a frame on which is mounted a drive mechanism that can be slidably moved along the longitudinal axis of the frame. The drive mechanism is adapted to rotate a drill string about its longitudinal axis. Sliding movement of the drive mechanism along the frame, in concert with the rotation of the drill string, causes the drill string to be longitudinally advanced into or withdrawn from the ground.
The length of the desired bore being drilled will vary according to the job at hand but may be substantial. In order to create a drill string of sufficient length to create the desired bore, many fixed lengths of drill rods may be attached end-to-end. More particularly, a first drill rod is placed on the machine rack and forced into the ground. A subsequent length of drill rod is placed on the machine and coupled to the first length, generally via threads on each drill rod. The combined length is then further forced into the ground. In order to form a complete bore, numerous drill rods are added in this fashion during the boring operation. As rods are added, the drill string length and the resulting bore length increases.
When two drill pipes are threaded together in the process of forming such a drill string, they are torqued to a predetermined torque (i.e., the makeup torque) to provide a secure connection. During drilling operations, the drill string is typically rotated in a forward direction (e.g., clockwise). Thus, assuming the pipes have right-hand threads, the forward rotation of the drill string encourages the pipes to remain threaded together. However, at times it is desirable to rotate the drill string in a reverse direction (e.g., counterclockwise). During this reverse rotation, the drill pipes are encouraged to become uncoupled. This is particularly true if the drill head of the drill string becomes wedged in hard soil or rock. It is important that the sections of drill pipe not become uncoupled. For example, if two of the drill pipes become uncoupled, a gap is formed in the threaded joint between the pipes that allows foreign matter to enter the joint. Until the foreign matter is removed, the matter can prevent the joint from being sufficiently retorqued. The loose joint will not be able to carry any reverse rotational torque load unless it is retorqued. If the uncoupling occurs underground, it may be difficult to identify that a joint has become loose and the operation and/or steering of the horizontal directional drilling machine can be negatively effected.
The situation is further complicated by the fact that the forces on pipe used for directional drilling are different than those encountered in vertical drilling operations. HDD pipe must be more flexible than pipe used in vertical drilling because it must bend in ways that are not required in vertical drilling. Pipe for HDD may be subject to more wear because it is supported by the bore wall during drilling and backreaming, and it may also encounter greater pullback and rotational forces than vertical pipe. Thus, the drill pipe utilized in these guided boring operations must be rigid enough to transmit torque, yet flexible enough to negotiate gradual turns as the direction of the bore hole changes. Generally, the flexibility of the drill pipe increases as the diameter of the pipe decreases. So, to improve flexibility, a smaller diameter pipe is preferred.
However, given the high working stresses at work in these operations, it is also true that as the diameter of the pipe, particularly in critical areas of the drill stem decreases, that the failure rate in these areas increases. To reduce likelihood of drill stem failure and yet provide good flexibility, current manufacturing methods include upsetting or expanding the ends of the shaft of the drill pipe by hot or cold forging techniques so larger diameter pin and box joints can be attached. The use of larger joints attached to the upset ends of smaller tubing has resulted in a more durable pipe design. However, the heat forging process typically used for deforming the ends of the drill pipe is time consuming and expensive because it requires high heat and multiple operations. Cold forging processes are also expensive.
Accordingly there is a need for a simpler and more economical method for providing an upset end of a tubular drill stem member used in HDD operations.
There is also a need to develop improved structures and techniques which allow the drill stem to be adequately torqued for any eventual situation which may be encountered in the drilling operation at hand.
A need may also exist to increase the overall rigidity of the joint between drill stem tubular members in some circumstances.